pkg-perl: fix for Perl XS packages configured by Makefile.PL

for native package, ExtUtils::MakeMaker detects all include filenames on system perl,
all these includes will be required by the generated Makefile but on STAGING_DIR.

patchlevel-debian.h is not a part of the Perl distribution, but it was added by some Linux distributions (Debian, Ubuntu, ...)
In this case, we create a dummy file on STAGING_DIR.